Sync via local IP: no problem. Sync via URL (via proxy webserver): not all files are synchronized

I created a new CentOS VM just for the latest Nextcloud version, and to try out PHP7.

The details of this fresh install:
CentOS 7.3.1611
PHP 7.1.2
MariaDB 5.5.52
Nextcloud 11.0.2

Windows sync client version: 2.2.4 (build 2).

To prevent problems I have experienced in the past, this is set also:
upload_max_filesize = 10G
post_max_size = 10G

Since I have only one external IP, I use a separate CentOS VM as a proxy server. Nextcloud is just one of the several VM’s behind this proxy server. (ProxyPass/ProxyPassReverse)
The SSL certificate for my Nextcloud installation can be found on the proxy server. All seems to work like it should. Except, it doesn’t.

If I use the Nextcloud sync client in Windows 10, and connect to the local Nextcloud IP (no SSL), ALL files sync without any problem.

If I use the Nextcloud sync client in Windows 10, and connect to the ‘official’ URL of my Nextcloud installation (with SSL), NOT all files are synchronized. PDF’s and JPG’s, for instance, are not synchronized.

Anyone any idea if my assumption that this somehow has to do with SSL, is correct?
If so, how to fix such a thing? If not, what then could be the cause?

Thx!

Don’t tell me I’m the only one that has encountered this problem :scream: